1. Field of the Invention
The present invention relates to an air/fuel ratio feedback control system for an internal combustion engine of a vehicle.
2. Description of Background Information
In an internal combustion engine provided with a three-way catalytic converter in the exhaust system, the feedback air/fuel ratio control system is designed to control the air/fuel ratio of the engine around a stoichiometric value (14.7:1 for example) in accordance with the composition of the exhaust gas and operational conditions of the engine. This is because an optimum operation of the catalytic converter is enabled around the stoichiometric value of the air/fuel ratio.
As this type of the air/fuel ratio control system, the present applicant has already proposed a type of the so called air intake side secondary air supply system. In the air intake side secondary air supply system of the applicant, there is provided an air control valve disposed in a secondary air supply passage communicated with the downstream of the throttle valve, for varying the sectional area of the secondary air supply passage in accordance with the pressure level in a pressure chamber of the air control valve. Further, the actual air/fuel ratio is detected from an oxygen content of the exhuast gas, and a first control pressure capable of opening the air control valve is applied to the pressure chamber of the air control valve, to gradually increase the sectional area of the secondary air supply passage when the detected air/fuel ratio is rich and a second control pressure capable of closing the air control valve is applied to the pressure chamber of the air control valve, to gradually decrease the sectional area of the secondary air supply passage when the detected air/fuel ratio is lean.
In the case of the internal combustion engine of a vehicle, during a deceleration, for example, the accelerator pedal may be released suddenly to cause an abrupt closing of the throttle valve. As the result, an excessive amount of fuel is sucked from the carburetor and applied to the engine cylinders due to an instantaneous increase of the vacuum level. Further, since the amount of the intake air is restricted by the throttle valve during deceleration, the air/fuel ratio becomes over rich due to the lack of required amount of the air, and the pressure in the engine cylinders goes down at the same time. Therefore, the temperature of combustion also goes down to cause unstable state of the combustion, which results in the production of an amount of noxious component, mainly HC (hydrocarbons) as unburnt component and CO (carbon monoxide), in the exhaust gas of the engine.
Conventionally, measures have been taken to suppress the discharge of this unburnt component, in which an exhaust side secondary air is introduced into the exhaust gas passage for the oxidization of the unburnt component during deceleration of the engine. However, in the case of the above mentioned air/fuel ratio control system, not only during the deceleration, the oxygen sensor produces a lean signal to indicate the condition of lean mixture when the exhaust side secondary air is supplied. This is because the supply of the secondary air is performed upstream of the oxygen sensor, and the air/fuel ratio of the mixture is falsely determined to be lean in accordance with the lean signal from the oxygen sensor. Therefore, the pressure chamber of the air control valve is supplied with the second control pressure to cause the closure of the air control valve. In other words, during deceleration, the air/fuel ratio is shifted to the rich side to stop the feedback control, and the control state turns to the open loop. After the deceleration, the supply of the exhaust side secondary air is stopped at the time of acceleration, and the opening degree of the air control valve is controlled in accordance with the oxygen sensor output signal, to start the feedback control once more.
However, it takes a time before detecting the rich air/fuel ratio by means of the output signal of the oxygen sensor when the supply of the exhaust side secondary air is stopped. Moreover, since the pressure value in the pressure chamber of the air control valve at the time of the cease of the supply of the exhaust side secondary air is substantially equal to the pressure level of the second control pressure, it also takes relatively long time before the air control valve is fully opened by the supply of the first control pressure into the pressure chamber in accordance with the output signal of the oxygen sensor after the supply of the exhaust side secondary air is stopped.
Due to this delay of response of the air control valve, the delay of the change of the air/fuel ratio toward the lean side is inevitable and the unburnt component such as HC and CO is generated as the result of the over rich air/fuel mixture.
Further, this problem is not limited in the state of deceleration of the vehicle in which the air/fuel ratio control is switched to the open loop control state, but also present in the other driving states of the vehicle in which the exhaust side secondary air is supplied when the open loop control is established, and the supply of the exhaust side secondary air is stopped at the same time of restarting the feedback control.